/etc/localtime

/etc/localtime configures the system-wide timezone of the local system.
It may be link to /usr/share/zoneinfo/ followed by a timezone identifier such as "Europe/Berlin" or "Etc/UTC"
pointing to binary tzfile(5) timezone data for the configured timezone.

Timezone identifier is extracted from the target name of /etc/localtime,

The timezone may be overridden for individual programs by using the $TZ . See environ(7).

timedatectl(1) changes this file during runtime.
systemd-firstboot(1) to initialize the time zone on mounted (but not booted) system images.